In the above preferred field, it is known to use a magnetic sensor adapted to measure the change in the intensity of a magnetic induction field when a ferromagnetic encoder fitted with field-disturbing members moves past a detector cell. The detector cell such as Hall effect probe or a magneto-resistive probe, for example, delivers a periodic sinewave signal. The detector cell is associated with a level comparator exhibiting hysteresis, such as a Schmitt trigger, so as to obtain clean transitions in its output voltage for values of the magnetic induction that are distinct depending on whether it is increasing or decreasing.
In order to make a sensor for detecting a speed, it is known to provide an encoder having teeth that are disposed in regular manner and in large number so as to improve the resolution of such a sensor. A known improvement to that kind of sensor consists in making an encoder constituted by a multipole magnetic ring, having alternating north poles and south poles provided around its circumference and regularly spaced apart at a given pitch.
In order to determine at least one position, e.g. corresponding to ignition top dead-center for a cylinder, it is known to provide the magnetic encoder with a reference. Thus, it is known to omit e.g. two teeth from the toothed wheel. In the solution making use of an encoder with alternating north poles and south poles, it could be envisaged either to omit a plurality of magnetic poles, thereby leaving an empty space, or else to replace one or more poles of given sign with one or more poles of opposite sign. A pole having magnetization of one sign is thus provided between two adjacent poles of opposite sign, which pole presents a spacing that differs from the pitch at which the other poles are spaced.
In order to obtain good measurement accuracy, particularly concerning detection of the irregular pole, patent FR 2 757 943 teaches making an encoder which comprises, for each irregular pole, means for correcting the value of the magnetic field created by the irregular pole so that the signal delivered by the passage of the poles adjacent to said irregular pole is symmetrical about the null value of the magnetic field.
Implementing such an encoder makes it possible to obtain a magnetic signal at the output from the detector cell of the sensor having a period that is constant concerning the regular poles. As a result, measurements performed in this way are of good accuracy, in particular when identifying the irregular pole.
In an embodiment described in that patent, the means for correcting the value of the magnetic field of the irregular pole are formed by the thickness profile of said irregular pole.
Although the technical solution described in that patent gives satisfaction in practice, the Applicant has devised a way of improving such an encoder so as to make it easier to manufacture and so as to make detection possible in differential mode, in spite of the fact that the magnetic signal delivered by the irregular pole of the above-described encoder presents points of inflection.